Kenwood KM600 motor doesn't operate when drive is mounted?

Hi,
I have a Kenwood KM600 that ceased to work some time ago. I replaced the control and it now works fine until I mount the drive belt.
Once the drive belt is mounted the motor doesn't spin. I assume the torque required is too high. Spinning the belt by hand works with out resistance in the gearbox, so it should work. The contraption can be spun by hand but not by the motor.
Assuming the torque needed is enough to hold the motor it would probably be that the rotor cannot have enough amps = resistance between coals and rotor. But this measures - as expected - close to zero ohms.
Any suggestions are welcome.
Ulf